[Fix] Prevent splitting UTF-8 sequences in MIME header encoding

diff --git a/src/libmime/mime_headers.c b/src/libmime/mime_headers.c
index 4e5aacd227fced3b786eba99f820b9f256725f37..5bffb801f20f8e583900edf423aa1d2af5fb7843 100644 (file)
--- a/src/libmime/mime_headers.c
+++ b/src/libmime/mime_headers.c
@@ -883,8 +883,14 @@ rspamd_mime_header_encode(const char *in, gsize len, bool is_structured)
                        size_t pending_ws_budget = include_pending_ws ? (pending_spaces + pending_tabs * 3) : 0;
 
                        /* Determine how much of this piece needs encoding and fits the budget */
+                       size_t utf8_char_start = 0;
+                       size_t enc_span_at_utf8_start = 0;
+                       size_t encoded_len_at_utf8_start = 0;
+
                        for (size_t i = 0; i < piece_len; i++) {
                                unsigned char c = p[i];
+                               /* UTF-8 lead byte or ASCII */
+                               gboolean is_utf8_start = (c < 0x80) || (c >= 0xC0);
 
                                if (!need_encoding) {
                                        if (c >= 128 || (is_structured && !g_ascii_isalnum(c))) {
@@ -901,6 +907,11 @@ rspamd_mime_header_encode(const char *in, gsize len, bool is_structured)
 
                                                encoded_len_count = pending_ws_budget + add;
                                                enc_span = 1;
+                                               if (is_utf8_start) {
+                                                       utf8_char_start = i;
+                                                       enc_span_at_utf8_start = 0;
+                                                       encoded_len_at_utf8_start = pending_ws_budget;
+                                               }
                                        }
                                        else {
                                                /* Still in unencoded prefix */
@@ -908,6 +919,12 @@ rspamd_mime_header_encode(const char *in, gsize len, bool is_structured)
                                        }
                                }
                                else {
+                                       if (is_utf8_start) {
+                                               utf8_char_start = i;
+                                               enc_span_at_utf8_start = enc_span;
+                                               encoded_len_at_utf8_start = encoded_len_count;
+                                       }
+
                                        /* Also stop on parentheses to keep CFWS outside */
                                        if (c == '(' || c == ')') {
                                                piece_len = i;
@@ -946,9 +963,18 @@ rspamd_mime_header_encode(const char *in, gsize len, bool is_structured)
                                        size_t add = (g_ascii_isalnum(c) || c == ' ') ? 1 : 3;
 
                                        if (encoded_len_count + add > max_token_size) {
-                                               /* Budget exceeded; stop encoded span before this char */
-                                               piece_len = i;
-                                               piece_end = p + piece_len;
+                                               /* Budget exceeded; stop at UTF-8 boundary */
+                                               if (is_utf8_start) {
+                                                       piece_len = i;
+                                                       piece_end = p + piece_len;
+                                               }
+                                               else {
+                                                       /* Back up to UTF-8 char start */
+                                                       piece_len = utf8_char_start;
+                                                       piece_end = p + piece_len;
+                                                       enc_span = enc_span_at_utf8_start;
+                                                       encoded_len_count = encoded_len_at_utf8_start;
+                                               }
                                                break;
                                        }